The invention relates to an electric lamp comprising:
a light-transmissive lamp vessel which is closed in a vacuumtight manner and has a quartz glass wall enclosing a space, accommodating an electric element; PA1 a metal foil completely embedded in the wall and having knife edges formed by knife planes; PA1 at least an internal current conductor which is connected to the embedded metal foil and projects into the space; PA1 at least an external current conductor which is connected to the embedded metal foil, projects from the wall of the lamp vessel and is provided with a coating.
A lamp of this type is known from U.S. Pat. No. 3,420,944. During operation of the known lamp, a part of the external current conductor and the metal foil, generally of molybdenum with an additive of, for example, 0.5-1.0% by weight of Y.sub.2 O.sub.3, has a temperature of more than 450.degree. C. In a lamp in which no measures were taken to inhibit corrosion of the external current conductor and the metal foil, these metal parts would corrode due to the high temperature in so far as the metal parts have an open connection with the atmosphere outside the lamp via a capillary around the external current conductor. Corrosion of the metal foil and/or the external current conductor leads to failure of the lamp due to the interruption of the current supply. The known lamp is protected against corrosion by providing, prior to its manufacture, a chromium coating on the external current conductor and at least parts of the metal foil, the knife edges and the knife planes. At locations where the coating is provided, the protection after manufacture of the lamp has remained intact, but the coating is partly converted into a chromium-containing protective coating. Both the coating and the protective coating retard the corrosion during operation of the lamp.
It is known that, in addition to corrosion of the current feed-through as a cause of premature failure of the lamp, there are various other causes of premature failure. Other causes may be, for example, leakage of the lamp vessel or, for example, an explosion of the lamp. The risk of failure of the lamp due to these other causes has appeared to be small in practice if the lamp has operated for less than a thousand hours.
The corrosion protection of the lamp as is known from U.S. Pat. No. 3,420,944 has the drawback that this leads to such a long lifetime of the lamp, for example, more than a thousand operating hours, that the risk of the lamp failure due to an explosion of the lamp and the risk of follow-up damage are unacceptably greater. The coating has a coating thickness and a quality level determining the corrosion protection and influencing the lifetime of the lamp. However, the quality level and the coating thickness in the known lamp are not controlled to such an extent that a lifetime limitation of a thousand operating hours is adjustable, which leads to an unacceptably large spread of the lamplife.
Another drawback of the known lamp is that the coating must be provided on the metal foil. Due to the extra treatments with the vulnerable metal foil, there is a great risk that the knife edges of the metal foil are damaged. The damaged knife edges of the metal foil embedded in the finished lamp lead to high tensions in the wall of the lamp vessel so that the risk of failure during manufacture of the lamp or due to premature leakage of the lamp vessel will be unacceptably greater.